Overvoltage Protection Device

ABSTRACT

An overvoltage protection device (100) may include a metal oxide varistor (MOV) (102) having a first surface (114) and a second surface (116); a semiconductor substrate (202) having a first outer surface (126) and a second outer surface (128) and comprising a semiconductor crowbar device (104) comprising a plurality of semiconductor layers arranged in electrical series to one another, the semiconductor substrate (202) being disposed on a first side of the metal oxide varistor (102), a conductive region (124) disposed between the second surface (116) of the MOV (102) and the first outer surface (126) of the semiconductor substrate (202); a first electrical contact (120) disposed on the first surface (114) of the MOV (102); and a second electrical contact (122) disposed on the second outer surface (128) of the semiconductor substrate (202).

BACKGROUND Field

Embodiments relate to the field of circuit protection devices, and moreparticularly to a semiconductor devices for protection againstovervoltage events.

Discussion of Related Art

Semiconductor devices are widely used to provide protection againsttransient conditions, such as transient overvoltage events or surgeevents, by taking advantage of the properties of P/N junctions. In thepresent day there are two main type of discrete circuit protectiontechnologies widely deployed in the market. These may be referred to ascrowbar devices and clamping devices. Examples of clamping devicesinclude varistors generally fabricated as metal oxide varistors (MOV),as well as Zener diodes. In either of these devices, voltage may beclamped to a level characteristic of the particular clamping device. Adrawback of the use of a clamping device is the relatively slow responsewith a high clamping voltage, often 1.6 times to 2.5 times the standoffvoltage. Additionally a high leakage current may be present in an MOVdevice and self-heat dissipation may accelerate aging and result in anMOV being unsuitable for withstanding multiple surge events. Crowbartype devices return to a lower voltage stage when a certain voltage isreached. One issue regarding the use of crowbar devices is that thesedevices do not return to a low leakage state in the absence ofresetting, or until current through the device returns to a low levelcharacteristic of their hold current.

It is with respect to these and other issues the present disclosure isprovided.

SUMMARY

In one embodiment, an overvoltage protection device may include a metaloxide varistor (MOV) having a first surface and a second surface, asemiconductor substrate having a first outer surface and a second outersurface and comprising a semiconductor crowbar device comprising aplurality of semiconductor layers arranged in electrical series to oneanother, the semiconductor substrate being disposed on a first side ofthe metal oxide varistor; a conductive region disposed between thesecond surface of the MOV and the first outer surface of thesemiconductor substrate; a first electrical contact disposed on thefirst surface of the MOV; and a second electrical contact disposed onthe second outer surface of the semiconductor substrate.

In another embodiment, a method of fabricating an overvoltage protectiondevice may include providing a metal oxide varistor (MOV) having a firstside and a second side; attaching a first surface of a semiconductorsubstrate comprising a semiconductor crowbar device to the second side;forming a first electrical contact on the first side of the metal oxidevaristor; and forming a second electrical contact on a second surface ofthe semiconductor substrate opposite the first surface, wherein themetal oxide varistor and the semiconductor crowbar device are inelectrical series with one another between the first electrical contactand the second electrical contact.

In an additional embodiment, an overvoltage protection device mayinclude a first electrical contact; a metal oxide varistor (MOV)electrically connected to the first electrical contact; a secondelectrical contact; and a semiconductor crowbar device comprising aplurality of semiconductor layers, wherein the metal oxide varistor andthe semiconductor crowbar device are arranged in electrical seriesbetween the first electrical contact and second electrical contact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A presents a circuit representation of one implementation of anovervoltage protection device according to embodiments of thedisclosure;

FIG. 1B presents a side cross-sectional view of a structure of anovervoltage protection device according to embodiments of thedisclosure;

FIG. 2 presents a side cross-sectional view of a semiconductor crowbardevice that may form part of an overvoltage protection device accordingto various embodiments;

FIG. 3A depicts an overvoltage protection device before encapsulationaccording to some embodiments;

FIG. 3B depicts the overvoltage protection device of FIG. 3A afterencapsulation;

FIG. 4A presents an exemplary current-voltage curve for an overvoltageprotection device according to embodiments of the disclosure;

FIG. 4B presents an exemplary current-voltage curve for a firstcomponent of an overvoltage protection device;

FIG. 4C presents an exemplary current-voltage curve for a secondcomponent of an overvoltage protection device;

FIG. 4D presents an exemplary breakover voltage behavior comparing adevice according to the present embodiments and standalone devices;

FIG. 5 presents an exemplary process flow; and

FIG. 6A to 6F presents a pictorial representation of an exemplaryovervoltage protection device at various stages of fabrication accordingto embodiments of the disclosure.

DESCRIPTION OF EMBODIMENTS

The present embodiments will now be described more fully hereinafterwith reference to the accompanying drawings, in which variousembodiments are shown. The embodiments may be embodied in many differentforms and are not to be construed as limited to the embodiments setforth herein. These embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theembodiments to those skilled in the art. In the drawings, like numbersrefer to like elements throughout.

In the following description and/or claims, the terms “on,” “overlying,”“disposed on” and “over” may be used in the following description andclaims. “On,” “overlying,” “disposed on” and “over” may be used toindicate when two or more elements are in direct physical contact withone another. The terms “on,”, “overlying,” “disposed on,” and over, mayalso mean when two or more elements are not in direct contact with oneanother. For example, “over” may mean when one element is above anotherelement and not in contact with another element, and may have anotherelement or elements in between the two elements. Furthermore, the term“and/or” may mean “and”, it may mean “or”, it may mean “exclusive-or”,may mean “one”, may mean “some, not all”, may mean “neither”, and/or itmay mean “both.” The scope of claimed subject matter is not limited inthis respect.

The present embodiments are generally related to overvoltage protectiondevices. In various embodiments, an overvoltage protection device maycomprise an integration of a metal oxide varistor (MOV) with asemiconductor crowbar device. As detailed below, such a device mayprovide the advantages of a low clamping voltage, low leakage, and fastresponse time. The semiconductor crowbar component of the overvoltageprotection devices of the present embodiments may provide excellentresponse to a surge, low leakage, durable protection while not wearingout, as well as accurate and consistent breakover voltage. The MOVcomponent of such overvoltage protection devices may provide high energyclamping to respond to a surge. As used herein, the term “semiconductorcrowbar device” may refer to a device that is implemented in asemiconductor substrate and acts to pull a voltage level down below atrigger level that is triggered in the event of an overvoltagecondition. When implemented as a single device, the trigger level in asemiconductor crowbar device may be represented as a breakover voltage.When the breakover voltage is reached, the semiconductor crowbar devicemay enter an ON state and act to pull down the voltage to a level suchas close to ground level. This action may be distinct from a clampingdevice such as a varistor, where voltage may be clamped to a clampingvoltage where the varistor becomes electrically conducting. Examples ofknown semiconductor crowbar devices include thyristor type devices and aSIDACtor® type device (SIDACtor is a registered trademark of Littelfuse,Inc.).

In various embodiments an overvoltage protection device is provided thatovercomes limitations of known MOV devices. In known MOV devices, forexample, the clamping voltage may range from approximately 1.6 time to2.5 times a working voltage. This may limit the ability of an MOV deviceto fully protect electronic equipment due to high energy that may betransmitted in the event of an overvoltage condition. In an overvoltageprotection device arranged according to the present embodiments, asemiconductor crowbar component may be coupled to an MOV in a mannerthat acts as a high impedance switch during an off-state. This helps tolower overall leakage current.

FIG. 1A presents a circuit representation of one implementation of anovervoltage protection device according to embodiments of thedisclosure. In particular, an overvoltage protection device 100 isimplemented between a first electrical line 110 and second electricalline 112. The first electrical line 110 and second electrical line 112may be coupled to an alternating current (AC) voltage source or a directcurrent DC voltage source.

In operation, the overvoltage protection device 100 may act to limitvoltage whether coupled to a DC source or AC source. In the example ofFIG. 1A, the overvoltage protection device 100 may protect a powercircuit 106 by limiting the voltage or energy that passes through thepower circuit 106 during an overvoltage event. In various embodimentsthe overvoltage protection device 100 may act as a bi-directional devicethat is a symmetrical device where the overvoltage protection device 100provides a first current-voltage (I-V) characteristic in response to apositive external voltage that is the same as a second current-voltagecharacteristic in response to a negative external voltage.

In operation according to known principles, a bi-directional orsymmetrical semiconductor crowbar device may provide an effective ACpowerline protection. Under normal operation when AC voltage does notexceed a breakover voltage, such a semiconductor crowbar device does notturn on. When AC peak voltage or a surge transient voltage exceeds thebreakover voltage, the semiconductor crowbar device may turn on, placingthe semiconductor crowbar device in a low voltage ON state, triggeringthe external transient voltage to be diverted.

Since there is an AC coupling when the semiconductor crowbar deviceturns on, the short duration of AC rise cycle may go into thesemiconductor protector. If this AC rise current does not exceed themaximum rated I_(TSM) (non-repetitive peak on state current) valuerepresenting the single AC cycle current withstand capability, thesemiconductor crowbar device may withstand the overvoltage conditionwhile not experiencing any degradation. When the sinusoidal cycle goesto the negative voltage portion of a cycle, a zero crossing isexperienced to reset the semiconductor crowbar device to an OFF-state,since the current decreases to below a value of the holding current ofthe semiconductor crowbar device. Thus, a semiconductor crowbar devicemay act as an AC line protection device to protect an AC transient surgeas well as an AC coupling event. In the embodiments as shown in FIG. 1A,when a semiconductor crowbar device is coupled together with a metaloxide varistor (MOV) device, the resulting overvoltage protection devicemay form a very low clamping voltage bi-directional surge protectiondevice.

FIG. 1B presents a side cross-sectional view of a structure of animplantation of the overvoltage protection device 100 according toembodiments of the disclosure. As illustrated, the overvoltageprotection device 100 may include the metal oxide varistor 102 and thesemiconductor crowbar device 104. The metal oxide varistor 102 mayinclude a first surface 114 and second surface 116. The metal oxidevaristor 102 may be formed of any known MOV material according to knownfabrication techniques. As detailed below, a semiconductor crowbardevice of the present embodiments may include a plurality ofsemiconductor layers that are arranged in electrical series within asemiconductor substrate. As suggested in FIG. 1B the semiconductorcrowbar device 104 may be implemented as a semiconductor substrate thathas a first outer surface 126 and second outer surface 128. In thisexample the semiconductor crowbar device 104 (semiconductor substrate)is disposed on a first side 130 of the metal oxide varistor 102.

A conductive region 124 is disposed between the second surface 116 ofthe metal oxide varistor 102 and the first outer surface 126 of thesemiconductor crowbar device 104. In different implementations asdiscussed below, the conductive region 124 may include a plurality oflayers or components serving to electrically connect the metal oxidevaristor 102 and semiconductor crowbar device 104. The conductive region124 may be formed by applying at least one coat (coating) and mayadditionally include sheet metal or other metallic layer to provideelectrical conductivity between the metal oxide varistor 102 andsemiconductor crowbar device 104.

The overvoltage protection device 100 may further include a firstelectrical contact 120 disposed on the first surface 114 of the metaloxide varistor 102, and a second electrical contact 122 disposed on thesecond outer surface 128 of the semiconductor crowbar device 104. Thefirst electrical contact 120 and second electrical contact 122 may beimplemented as metallic pieces including a thin metallic plate or sheetmade of material such as copper in one example. As shown in FIG. 1B, themetal oxide varistor 102 and semiconductor crowbar device 104 mayaccordingly be disposed in electrical series between the firstelectrical contact 120 and second electrical contact 122.

As detailed below in some implementations an overvoltage protectiondevice according to various embodiments may include electrical leads(not shown in FIG. 1B) attached to the first electrical contact 120 andsecond electrical contact 122. This may allow for convenient handlingand implementation as protection devices for circuits or electricalcomponents. For example, it may be useful to encapsulate electricalcomponents such as the semiconductor crowbar device 104 and metal oxidevaristor 102 with an electrically insulating layer. The use ofelectrical leads may accordingly provide a convenient means to accessthe overvoltage protection device 100 when encapsulated by an electricalinsulator.

FIG. 2 presents a side cross-sectional view of a semiconductor crowbardevice 200 that may form part of an overvoltage protection deviceaccording to various embodiments. As illustrated the semiconductorcrowbar device 200 is formed within a semiconductor substrate 202, suchas monocrystalline silicon. The semiconductor substrate 202 may includean inner n-type layer 204, a first outer n-type layer 206, a secondouter n-type layer 208, a first p-type layer 210 disposed between thefirst outer n-type layer 206 and inner n-type layer 204, and a secondp-type layer 212 disposed between the second outer n-type layer 208 andinner n-type layer 204. In particular, the first n-type outer layer 208may be disposed on a first outer surface 220 of the semiconductorsubstrate 202, while the second n-type outer layer 208 is disposed on asecond outer surface 222 of the semiconductor substrate 202.

Although not shown in FIG. 2, to form an integrated overvoltageprotection device according to various embodiments, the semiconductorsubstrate 202 may be implemented as the semiconductor crowbar device 104of FIG. 1B. Thus, an MOV device may be disposed on one side of thesemiconductor substrate 202, such as adjacent the first outer surface220 or adjacent the second outer surface 222. For example, The MOVdevice and semiconductor substrate may be electrically coupled togetheras illustrated in FIG. 1B. In addition, electrical contacts may beformed on an outer surface of the semiconductor substrate 202 as well ason an outer surface of the MOV as also illustrated in FIG. 1B. In thismanner, an MOV may be placed in electrical series with the variouslayers of the semiconductor crowbar device 200.

As further illustrated in FIG. 2, a first portion of the first p-typelayer 210 may be disposed on the first outer surface 220 and a secondportion of the second p-type layer 212 may be disposed on the secondouter surface 222. A first external contact (not shown) to the firstouter surface 220 of the crowbar device 200 may contact the first p-typelayer 210 and the first outer n-type layer 206 in electrically parallelfashion. Likewise a second external contact to the second outer surface222 of the crowbar device 200 may contact the second p-type layer 212and the second outer n-type layer 208 in electrically parallel fashion.According to various embodiments, in response to an overvoltage event,the semiconductor crowbar device 200 may react similarly, notnecessarily identically, to a known standalone semiconductor crowbardevice. For example, the semiconductor crowbar device 200, whenimplanted in series with a metal oxide varistor, may exhibit a breakovervoltage in response to an external voltage event. In particular, thesemiconductor crowbar device 200 may be placed in an ON state when anexternal voltage experienced between the first outer surface 220 andsecond outer surface 222 equals or exceeds the breakover voltage, inaccordance with known behavior of semiconductor crowbar devices.

FIG. 3A depicts an overvoltage protection device 300 according to someembodiments. In this example, the overvoltage protection device 300 isshown before encapsulation. The overvoltage protection device 300 mayinclude a metal oxide varistor 302 arranged in electrical series with asemiconductor crowbar device 304. A conductive region 312 may bedisposed between the metal oxide varistor 302 and semiconductor crowbardevice 304 to electrically couple the metal oxide varistor 302 andsemiconductor crowbar device 304. A conductive layer 306 may be disposedon an outer surface of the semiconductor crowbar device 304 as shown. Anelectrical contact 308 may be adjoined to the electrically conductivelayer 306. An electrically conductive layer 310 may also be disposed onan outer surface of the metal oxide varistor 302. Additionally a firstelectrical lead 314 may be connected to the electrically conductivelayer 310, while a second electrical lead 316 is connected to theelectrical contact 308.

FIG. 3B depicts the overvoltage protection device of FIG. 3A afterencapsulation, in this case represented as the overvoltage protectiondevice 320. In this example, an electrically insulating coating 322 isdisposed so as to encapsulate the metal oxide varistor 302 andsemiconductor crowbar device 304. The overvoltage protection device 320may thus be conveniently integrated into protection circuits forprotecting target devices or other components by adjoining the firstelectrical lead 314 and second electrical lead 316 to differentelectrical lines that are placed at different potentials.

In various embodiments an overvoltage protection device may be arrangedhaving a metal oxide varistor and semiconductor crowbar device, whereinthe metal oxide varistor comprises a first standoff voltage, thesemiconductor crowbar device comprises a second standoff voltage, andthe overvoltage protection device comprises a total standoff voltageequal to a sum of the first standoff voltage and the second standoffvoltage. The semiconductor crowbar device and metal oxide varistor mayalso be arranged to place the overvoltage protection device in an ONstate when an external voltage exceeds a threshold value and furtherarranged to place the overvoltage protection device in an OFF state whenthe external voltage is below the threshold value.

Moreover, an overvoltage protection device may be arranged wherein theovervoltage protection device comprises a symmetrical device comprisinga first current-voltage characteristic in response to a positiveexternal voltage of a first magnitude applied between a first electricalcontact and second electrical contact, and comprising a secondcurrent-voltage characteristic in response to a negative externalvoltage of the first magnitude applied between the first electricalcontact and a second electrical contact, the second current-voltagecharacteristic matching the first current-voltage characteristic.

FIG. 4A presents an exemplary current-voltage curve, shown as curve 400,for an overvoltage protection device according to embodiments of thedisclosure. The curve 400 exhibits symmetrical behavior, including apositive voltage portion 402 and negative voltage portion 404. Theembodiments are not limited in this context. The curve 400 results fromthe combined action of an overvoltage protection device, such as thosedepicted in the FIGS. 1A, 1B, 3A, and 3B. For example, referring also toFIG. 3B, when an external voltage is experienced between the firstelectrical lead 314 and second electrical lead 316, the resultingcurrent that flows through a device such as overvoltage protectiondevice 320, is shown by the curve 400. In particular the curve 400 ischaracterized by a breakover voltage V_(BR). When the external appliedvoltage exceeds the V_(BR) voltage as represented by point B, thevoltage “folds back” to point C, where the overvoltage protection deviceclamps the external voltage to the level D representing the maximumclamping voltage. The breakover current ImA may be about 400 mA,characteristic of a semiconductor crowbar device. Notably, the curve 400is characteristic of an integrated overvoltage protection that resets toan OFF state once the external voltage event subsides. In addition, theOFF state leakage current of the curve 400 has a much lower value due tothe high impedance of the semiconductor crowbar device, in comparison asimple MOV device.

By way of comparison, FIG. 4B presents an exemplary current-voltagecurve, shown as curve 410, for a semiconductor crowbar device that maybe used in an overvoltage protection device of the present embodiments.The curve 410 represents the current-voltage characteristic of asemiconductor crowbar device when implemented as a stand-alonecomponent. In this example, when the external voltage reaches Vs thesemiconductor crowbar device enters an ON state and voltage reduces to alow level. FIG. 4C presents an exemplary current-voltage curve, shown ascurve 420, for an MOV device that may be used in an overvoltageprotection device of the present embodiments. When external voltagereaches a value Vnom, the MOV device becomes conducting and voltage isclamped. Although not expressly shown in curve 400, the response time ofan overvoltage protection device that includes an MOV and semiconductorcrowbar device arranged in electrical series fashion between twoelectrical lines may act much faster than a standalone MOV in responseto a transient voltage surge.

Table I. illustrates a comparison of experimental electrical data for anintegrated overvoltage protection device of the present embodiments incomparison to a standalone MOV and standalone SIDACtor device. Asillustrated, the device of the present embodiments improves clampingvoltage from 850V to 492V in comparison to a standalone MOV. For leakagecurrent (Idrm/Irrm), an improvement from 7.1 uA at 311 V for astandalone MOV to 0.25 uA at 311 V for a device of the presentembodiments is measured. As observed in Table I. a standalone SIDACtorprovides relatively low leakage and low clamping voltage for the sametest conditions, while having a large AC follow-on current. AC Follow oncurrent is a quantity characteristic of crowbar devices such as aSIDACtor. The rating for the follow on current for known SIDACtordevices ranges around the 230-270 A range (large). When a crowbar device(SIDACtor) is integrated with an MOV where the integrated device isclamping in nature, the integrated device exhibits a zero to a verysmall and short duration of follow current (less than 10 A). This almostzero AC follow on current is “absorbed” by the high impedance of theclamping device very quickly so the current subsides very quickly beforebuilds up.

TABLE I IDRM/IRRM Vclamp Ac follow VRMS (311 v) (Typ) on current MOV 220v 7.1 uA 850 v No Present 220 v 0.25 uA 492 v No/Small EmbodimentSIDACtor 220 v 0.19 uA <30 v Large

FIG. 4D presents an exemplary breakover voltage behavior comparing adevice according to the present embodiments and standalone devices. Asillustrated, the breakover voltage for a standalone SIDACtor, MOV, anddevice according to the present embodiments are plotted as a function ofrate of change in voltage with time (dV/dT). As shown the SIDACTorstandalone device shows a relatively flat response as a function ofdV/dT, while the MOV breakover voltage is sensitive to dV/dT andincreases dramatically above 100 V/μs, while the device according to thepresent embodiments shows an intermediate sensitivity of breakovervoltage to dV/dT. Accordingly, the present embodiments provide a fasterresponse to a voltage impulse than a conventional MOV device.

FIG. 5 presents an exemplary process flow 500. The process flow 500 maybe used for fabrication of an overvoltage protection device according tovarious embodiments of the disclosure. At block 502, a metal oxidevaristor is provided where the metal oxide varistor has a first side andsecond side. The metal oxide varistor may be fabricated from knownmaterial according to know processing techniques. The metal oxidevaristor (MOV) may have a planar structure having a target shape such asrectangular, circular, or elliptical. The embodiments are not limited inthis context.

At block 504, a first surface of a semiconductor substrate is attachedto the second side of the MOV. The semiconductor substrate may include asemiconductor crowbar device having multiple doped semiconductor layers.In some examples the semiconductor crowbar device may be fabricatedwithin a monocrystalline silicon substrate. The first surface of thesemiconductor substrate may be attached by applying an electricallyconductive material on the second side of the MOV, on the first surfaceof the semiconductor substrate, or on the second side of the MOV and thefirst surface of the semiconductor substrate. In some examples multipledifferent electrically conductive components including a coating such asconductive solder paste, such as a low temperature solder paste,metallic sheet, and the like, may be used to attach the MOV to thesemiconductor substrate.

At block 506, a first electrical contact is formed on the second side ofthe MOV, where the second side may be opposite the first side. The firstelectrical contact may include a metal sheet or plate, a conductivepaste or other components.

At block 508, a second electrical contact is formed on a second surfaceof the semiconductor substrate opposite the first surface. The firstelectrical contact may include a metal sheet or plate, a conductivepaste or other components. In this manner, the semiconductor crowbardevice and MOV may be in electrical series with one another between thefirst electrical contact and the second electrical contact.

Turning now to FIGS. 6A to 6F there is shown a pictorial representationof an exemplary overvoltage protection device at various stages offabrication, according to embodiments of the disclosure. In FIG. 6A toFIG. 6C a device is shown in plan view at early stages of fabrication.In FIG. 6A an MOV 602 is shown. The MOV 602 may have a circular shape inplan view, while other shapes are possible according to additionalembodiments. A coat such as a conductive paste 604 may be applied to asurface of the MOV 602. Subsequently, a flat copper piece 606 (copperslug) may be applied to the conductive paste 604 as shown. The flatcopper piece 606 may be formed from copper sheet, for example. The flatcopper piece 606 may have an area that is smaller than the area of theMOV 602 as shown.

In FIG. 6B there is shown a subsequent stage when a conductive paste608, which may be a solder paste, is applied to the outer surface of theflat copper sheet 606 away from the MOV 602. Subsequently, asemiconductor crowbar device 610 is attached to the conductive paste 608as shown in FIG. 6B. The semiconductor crowbar device 610 may beembodied as a rectangular silicon chip having similar dimensions to theflat copper sheet 606.

In FIG. 6C there is shown a subsequent stage of fabrication where aconductive paste 612 may be applied to a surface of the semiconductorcrowbar device 610. Subsequently, a flat copper piece 614 may be appliedto the conductive paste 612 as shown. The flat copper piece 614 may havean area having similar dimensions to the semiconductor crowbar device610. Together the components may form a device stack 620.

Subsequently the device stack 620 at the stage shown in FIG. 6C may beexposed to a low temperature anneal such as by hot air gun, where thetemperature of hot air in the hot air gun is above room temperature.This low temperature anneal may promote bonding of the stack ofcomponents disposed on the MOV 602.

Turning now to FIG. 6D there is shown a top angled view of multipleovervoltage protection devices at a subsequent stage of fabrication. Atthis stage, a pair of electrical leads, shown as an electrical lead 622,may be attached to opposite sides of the device stack 620, as shown. Inparticular, a conductive paste 624 may be used to attach an electricallead 622 to the flat copper piece 614. Another electrical lead may besimilarly attached to the side of the MOV 602 opposite the side wherethe semiconductor crowbar device 610 is situated. In particular, theother electrical lead may be attached to the opposite side forming anunannealed device before the low temperature anneal.

Turning now to FIG. 6E, there is shown a side view of the overvoltageprotection device at a later stage where an insulating sleeve isarranged around an electrical lead 622. Finally, at FIG. 6F there isshown side view at a stage where an insulating coating 626, such as aplastic coating, is formed so as to encapsulate the device stack 620 aswell as portions of electrical leads 622 adjacent the device stack 620.The insulating coating 626 may be formed by known methods forencapsulating varistors, for example. In one instance powder may beapplied to the device stack and heat introduced to form a continuouscoating.

While the present embodiments have been disclosed with reference tocertain embodiments, numerous modifications, alterations and changes tothe described embodiments are possible while not departing from thesphere and scope of the present disclosure, as defined in the appendedclaims. Accordingly, the present embodiments may not be limited to thedescribed embodiments, and have the full scope defined by the languageof the following claims, and equivalents thereof.

What is claimed is:
 1. An overvoltage protection device, comprising: ametal oxide varistor (MOV) having a first surface and a second surface;a semiconductor substrate having a first outer surface and a secondouter surface and comprising a semiconductor crowbar device comprising aplurality of semiconductor layers arranged in electrical series to oneanother, the semiconductor substrate being disposed on a first side ofthe metal oxide varistor; a conductive region disposed between thesecond surface of the MOV and the first outer surface of thesemiconductor substrate; a first electrical contact disposed on thefirst surface of the MOV; and a second electrical contact disposed onthe second outer surface of the semiconductor substrate.
 2. Theovervoltage protection device of claim 1, wherein the metal oxidevaristor and the semiconductor crowbar device are in electrical seriesbetween the first electrical contact and the second electrical contact.3. The overvoltage protection device of claim 1, wherein the metal oxidevaristor comprises a first standoff voltage, the semiconductor crowbardevice comprises a second standoff voltage, and the overvoltageprotection device comprises a total standoff voltage equal to a sum ofthe first standoff voltage and the second standoff voltage.
 4. Theovervoltage protection device of claim 1, wherein the metal oxidevaristor comprises a first response time as a standalone device, andwherein the overvoltage protection device comprises a second responsetime less than the first response time.
 5. The overvoltage protectiondevice of claim 1 wherein the metal oxide varistor comprise a firstleakage current at a first voltage as a standalone device, and whereinthe overvoltage protection device comprises a second leakage current atthe first voltage that is less than the first leakage current.
 6. Theovervoltage protection device of claim 1, the semiconductor crowbardevice and metal oxide varistor arranged to place the overvoltageprotection device in an ON state when an external voltage exceeds athreshold value and further arranged to place the overvoltage protectiondevice in an OFF state when the external voltage is below the thresholdvalue.
 7. The overvoltage protection device of claim 1, wherein thesemiconductor crowbar device comprises: an inner n-type layer; a firstouter n-type layer disposed on the first outer surface; a second outern-type layer disposed on the second outer surface; a first p-type layerdisposed between the first outer n-type layer and the inner n-typelayer; and a second p-type layer disposed between the second outern-type layer and the inner n-type layer.
 8. The overvoltage protectiondevice of claim 7, wherein a first portion of the first p-type layer isdisposed on the first outer surface and a second portion of the secondp-type layer is disposed on the second outer surface.
 9. The overvoltageprotection device of claim 1, wherein the conductive region comprises: afirst conductive layer disposed on the second surface of the metal oxidevaristor; a second conductive layer disposed on the first outer surfaceof the semiconductor substrate; and a first copper slug disposed betweenthe first conductive layer and the second conductive layer.
 10. Theovervoltage protection device of claim 1, wherein the second electricalcontact comprises: a third conductive layer disposed on the second outersurface of the semiconductor substrate; and a second copper slugdisposed on the third conductive layer.
 11. The overvoltage protectiondevice of claim 1, wherein the overvoltage protection device comprises asymmetrical device comprising a first current-voltage characteristic inresponse to a positive external voltage of a first magnitude appliedbetween the first electrical contact and second electrical contact, andcomprising a second current-voltage characteristic in response to anegative external voltage of the first magnitude applied between thefirst electrical contact and second electrical contact, the secondcurrent-voltage characteristic matching the first current-voltagecharacteristic.
 12. A method of fabricating an overvoltage protectiondevice, comprising: providing a metal oxide varistor (MOV) having afirst side and a second side; attaching a first surface of asemiconductor substrate comprising a semiconductor crowbar device to thesecond side; forming a first electrical contact on the first side of themetal oxide varistor; and forming a second electrical contact on asecond surface of the semiconductor substrate opposite the firstsurface, wherein the metal oxide varistor and the semiconductor crowbardevice are in electrical series with one another between the firstelectrical contact and the second electrical contact.
 13. The method ofclaim 12, wherein the attaching comprises: applying a first coatcomprising a low temperature solder paste to a first surface of themetal oxide varistor (MOV); attaching a first metallic piece to thefirst coat; applying a second coat comprising the low temperature solderpaste to the first metallic piece; and attaching the semiconductorsubstrate to the second coat.
 14. The method of claim 12, wherein theforming the second electrical contact comprises: applying a third coatcomprising a low temperature solder paste to the semiconductorsubstrate; and attaching a second metallic piece to the third coat toform an unannealed device.
 15. The method of claim 14, furthercomprising applying hot air to the unannealed device at a firsttemperature.
 16. The method of claim 14, further comprising: attaching afirst electrical lead to the first side of the metal oxide varistor; andattaching a second electrical lead to the second metallic piece.
 17. Themethod of claim 12, further comprising: applying a plastic coating tothe semiconductor substrate, the metal oxide varistor, the firstelectrical contact, and the second electrical contact.
 18. Anovervoltage protection device, comprising: a first electrical contact; ametal oxide varistor (MOV) electrically connected to the firstelectrical contact; a second electrical contact; and a semiconductorcrowbar device comprising a plurality of semiconductor layers, whereinthe metal oxide varistor and the semiconductor crowbar device arearranged in electrical series between the first electrical contact andsecond electrical contact.